Electrochemical sensor with thin film guard electrode

ABSTRACT

There is presented an electrochemical sensor (100) for sensing an analyte in an associated volume (106), the sensor comprising a first solid element and a second solid element being joined to the first solid element, a chamber (110) being placed at least partially between the first solid element and the second solid element, said chamber comprising a reaction region (130), and a reservoir region (132) being connected with the reaction region, wherein an one or more analyte permeable openings (122) connect the reaction region (130) with the associated volume (106), and wherein the electrochemical sensor (100) further comprises an analyte permeable membrane (124) in said one or more analyte permeable openings, a working electrode (104) a reference electrode (108), and a guard electrode (109) arranged so as to enable reduction or oxidation of at least some reactants from at least a part of the reservoir region.

FIELD OF THE INVENTION

The present invention relates to an electrochemical sensor, and moreparticularly to an electrochemical sensor with thin film guard electrodeand a corresponding use.

BACKGROUND OF THE INVENTION

For numerous purposes, such as environmental monitoring, biologicalresearch, or wastewater treatment, it is beneficial to be able to senseor quantitatively measure the partial pressure or concentration of ananalyte in an associated volume. This can be the partial pressure of agas in a gas atmosphere or the concentration of a dissolved gas in avolume of liquid.

Electrochemical sensors for sensing an analyte in an associated volumehave previously been proposed, but may be seen as being slow (such asslow during start-up and/or having long response times), fragile, bulky,flow dependent, unstable (such as having high baseline drift), difficultto manufacture, and/or insensitive (with respect to the analyte).

An improved electrochemical sensor for sensing an analyte in anassociated volume would be advantageous, and in particular a sensorpresenting an improvement on one or more of the parameters mentionedabove, would be advantageous.

SUMMARY OF THE INVENTION

It may be seen as an object of the present invention to provide anelectrochemical sensor that solves the above mentioned problems of theprior art.

It is a further object of the present invention to provide analternative to the prior art.

Thus, the above described object and several other objects are intendedto be obtained in a first aspect of the invention by providing anelectrochemical sensor, such as a said sensor being microfabricatedand/or being a microsensor, for sensing an analyte in an associatedvolume, the sensor comprising

-   -   a first solid element,    -   a second solid element being joined to the first solid element,    -   a chamber, such as a chamber for comprising an electrolyte, such        as an electrolyte solution, such as a chamber comprising an        electrolyte, such as a chamber comprising an electrolyte        solution, said chamber being placed at least partially between,        such as fully between, the first solid element and the second        solid element, said chamber comprising        -   a reaction region, and        -   a reservoir region being connected, such as fluidically            connected, with the reaction region,            wherein one or more analyte permeable openings connect the            reaction region with the associated volume, such as said one            or more analyte permeable openings forming a diffusion            barrier between the associated volume and the chamber, and            wherein the electrochemical sensor further comprises    -   an analyte permeable membrane in said one or more analyte        permeable openings, such as a silicone membrane, such as a        membrane which enables separating liquids (such as aqueous        solutions) on either side of the one or more analyte permeable        openings,    -   a working electrode in the reaction region, such as in between        the first solid element and the second solid element,    -   a reference electrode, and    -   a guard electrode arranged so as to enable reduction or        oxidation, such as reducing or oxidizing at the guard electrode,        of at least some reactants from at least a part of the reservoir        region, such as reactants which could otherwise diffuse to the        working electrode and be reduced or oxidized at the working        electrode (such as interfere with the sensing of the analyte),        wherein the guard electrode comprises a thin film, such as a        thin film placed on an inner wall of the chamber.

The invention is particularly, but not exclusively, advantageous forobtaining an electrochemical sensor, which may be provided in arelatively simple manner, such as an automated manner and/or viamicrofabrication since the chamber is placed at least partially between,such as fully between, first and second solid elements. The sensor mayfurthermore be robust due to the incorporation of the guard electrode asa thin film electrode since the thin film enables efficiently fixing theguard to an inner wall of the chamber. The sensor may furthermore beseen as enabling a high level of stability over extended periods of timesince the incorporation of the guard as a thin film enables bothlong-time operation due to efficient exchange of electrolyte species,such as ions, at the working electrode (in the reaction region) and lowdrift by having reactants from the reservoir effectively reduced oroxidized when they pass the guard electrode. Furthermore, the guardelectrode may ensure fast start-up, since it enables cleaning up thereservoir region of the chamber, so that a false-positive signal fromreactants diffusing from the reservoir region is effectively reduced oreliminated.

A possible advantage of the sensor may be that a critical part of thesensor is robust, i.e. tolerant to mechanical shocks. Specifically, thesensor can pass the transit drop test method 516.6 (shock) described instandard MIL-STD-810G, which includes 26 drops from a height of 4 feet(1.22 meters) on a drop zone of two inches of plywood over concrete.

By ‘electrochemical sensor’ is understood a sensor that detects thepresence, such as measure the concentration, of an analyte (such as anyone of nitrous oxide (N₂O), hydrogen sulfide (H₂S), oxygen (O₂),hydrogen (H₂), nitric oxide (NO)), by oxidizing or reducing the analyte(or a shuttle/mediator molecule) at the working electrode and detecting,such as measuring, the resulting current. It is understood that theresulting current need not necessarily be measured as a current, but mayfor example be measured as a voltage drop across a resistor. The words‘sensor’ and ‘electrochemical sensor’ are generally used interchangeablywithin the context of the present application. In embodiments theelectrochemical sensor comprises a voltage source, and a current meter.

By ‘microfabricated’ may be understood fabricated using one or more offour processes (comprising photolithography, thin-filmgrowth/deposition, etching, and bonding) to create objects with one ormore dimensions, such as at least one dimension decisive forperformance, in the range of nanometer to micrometers, such as within 1nanometer to 1 millimeter. In a microfabrication process, one may take asubstrate and build a device out of its bulk material and/or on itssurface. An example of a microfabricated sensor is given in thereference “Determination of blood pO ₂ using a micromachined Clark-typeoxygen electrode” by Hiroaki Suzuki et al., Analytica Chimica Acta 431(2001) 249-259, which reference is hereby incorporated by reference inentirety.

In an embodiment the sensor is a microsensor. By ‘microsensor’ may beunderstood a sensor with one or more dimensions, such as at least onedimension decisive for performance, in the range of micrometers, such aswithin 1 nanometer so 1000 micrometers, such as within 1 nanometer towithin 500 micrometers, such as within 1 nm to within 300 micrometers,such as within 1 nanometer to 100 micrometers. The dimension in therange of micrometers may be referred to as a characteristic length. Thedimension in the range of micrometers may be a diameter of an opening(or a maximum distance from a point in an inlet opening to the side ofthe inlet opening) or a distance from outside of the membrane (in theassociated volume) through the membrane and to the working electrode.

By ‘analyte’ is understood the compound of interest, such as a molecule,such as N₂O, H₂S, O₂, H₂, or NO.

By ‘sensing’ is understood qualitatively detecting the presence of ananalyte and/or quantitatively determining a partial pressure orconcentration of analyte in the associated volume. In some more specificembodiments, sensing may be construed as quantitatively measuring apartial pressure of an analyte. It is understood that quantifyingcomprises qualifying.

By ‘associated volume’ is understood an associated volume which isadjacent the sensor and which may contain the analyte. The associatedvolume is not to be construed as limiting to the scope of the claims.The partial pressure of the analyte in the associated volume may bemeasured with the sensor. If the associated volume comprise a liquid,the concentration of analyte is related to the partial pressure via thesolubility of the analyte. In order to avoid measuring the solubility,the sensor can be calibrated in solutions with known concentration. Theassociated volume may comprise a fluid, a gas or a matrix, such as anyone of a biofilm, an extracellular matrix, a solid-liquid matrix (suchas a sand-water matrix) and a solid-gas matrix (such as a sand-airmatrix). The associated volume may be understood to start at theopposite side of the analyte permeable membrane with respect to theworking electrode.

By ‘chamber’ is understood a chamber as is known in the art, such as acasing, which delimits a volume within the chamber from the surroundingsexternal to the chamber. However, it is encompassed by the presentinvention, that the chamber may have one or more through-going holes inthe delimiting walls, such as openings for filling or replacingelectrolyte solution or openings for electrical wiring or membranes.However, in general, the chamber does not allow fluid passage, such asuncontrolled fluid passage, from outside the chamber to inside thechamber. The chamber may be suitable for comprising an electrolytemedium. ‘Electrolyte medium’ is an electrically conducting medium inwhich the flow of current is accompanied by the movement of matter inthe form of ions, within which the analyte can diffuse. It is understoodthat the electrolyte medium electrically connects the referenceelectrode with ionic conductivity and the working electrode. Theelectrolyte medium may for example be an electrolyte solution (such as aliquid, such as a liquid which may ensure supply of ions to the workingelectrode where the analyte reacts for neutralizing reaction products),a gel electrolyte, a solid electrolyte or a paste electrolyte. By‘electrolyte solution’ is understood a liquid comprising ions, whereinthe charge carriers are dissolved ionic compounds. The chamber mayencompass a part of the one or more analyte permeable openings, such asthe part of the one or more analyte permeable openings on the chamberside of the analyte permeable membrane also being part of the chamber.Thus, the part of the analyte permeable openings which is on the chamberside of the analyte permeable membrane (such as on the opposite side ofthe analyte permeable membrane with respect to the associated volume)may be considered both part of the chamber and the one or more analytepermeable openings.

By ‘reaction region’ is understood a region of the chamber which iscloser to the working electrode than the guard electrode.

By ‘reservoir region’ is understood any portion of the chamber outsideof the reaction region, such as defined as the part of the chamber,which is closer to the guard electrode than the working electrode.

The feature ‘the reservoir region being connected, such as fluidicallyconnected, with the reaction region’ may be understood to specify, thations, such as counter ions in the electrolyte, may diffuse from thereservoir region to the reaction region. This may be enabled by havingthe reservoir region and the reaction region being fluidically connectedand/or connected by an electrolyte medium, such as an electrolytesolution or a gel electrolyte or a paste electrolyte or a solidelectrolyte.

By ‘first solid element’ and ‘second solid element’ may be understoodmay be understood solid elements (which may each be a multilayerelement) which may be joined at a common interface so as to encapsulatebetween them the chamber. The first and/or second solid element may beplanar, such as the interface between them being planar. The firstand/or second solid element may be formed in any one of silicon orglass. It is to be understood that the first and/or second solid elementmay comprise layers of materials, such as oxide or nitride layers, wherethe layers are then part of the first and/or second solid element. It isencompassed that the first solid element and/or the second solid elementcomprise a layer, such as a layer joined to another layer within thefirst or second solid element, that comprises the structure that formsthe cavity for the chamber and the one or more openings once the firstand second solid element are joined. The chamber and or the one or moremembrane permeable openings may be at least partially formed as one ormore non-through going holes in the first solid element and/or thesecond solid element.

By ‘joined’ may be understood that the first and second solid elementare joined together, such as forming a fluid-tight bond at a commoninterface (where it is understood that the fluid-tight bond may comprisedistinct openings, such as the one or more analyte permeable openingsand/or one or more openings for electrolyte filling). Forming afluid-tight bond may also be referred to as sealing the chamber byjoining of the first and second solid element so as to form a sealinginterface.

By ‘said chamber being placed at least partially between the first solidelement and the second solid element’ may be understood that while atleast some of the chamber may be placed between the first and secondsolid element, it is encompassed by the present invention, that thechamber may also comprise a volume extending beyond a volume between thefirst and second solid element. For example, in an embodiment, a casingis arranged for comprising a volume (an external reservoir, such aexternal to a volume between the first solid element and the secondsolid element) of the chamber which is outside a region between thefirst and second solid elements. An advantage of this may be that itenables forming delicate structures in small dimensions on small firstand second solid elements, while still enabling having a large chambervolume, which may be arranged for holding a relatively large electrolytevolume (compared to a volume between the first and second solidelements), which may in turn enable extending a life time of the sensor.

By ‘one or more analyte permeable openings’ is understood one or moreanalyte permeable through-going holes in the structure around thechamber, which connect the associated volume with a volume within thechamber, such as directly connect the associated volume with thereaction region. By ‘directly connect’ may be understood that

-   -   the reaction region is immediately adjacent to, such as abutting        or overlapping, one side of the one or more analyte permeable        openings and the associated volume may be immediately adjacent        to the opposite side of the one or more analyte permeable        openings, such that an analyte passing from the associated        volume to the reaction region via the one or more analyte        permeable openings need not pass the reservoir region,    -   and/or that    -   the shortest distance from        -   any point on the working electrode, through the one or more            analyte permeable openings and the analyte permeable            membrane to        -   a point on the opposite side of the analyte permeable            membrane with respect to the working electrode,    -   is equal to or less than 300 micrometer.

It may be understood that the one or more analyte permeable openingsform a diffusion barrier for analytes diffusing from the associatedvolume to the reaction region. The extent of an opening within the oneor more analyte permeable openings may be understood to be a volumewithin the opening along a path through the respective opening, such asa path through the opening which is parallel with a direction of flowthrough the hole and which path intersects the middle (such ascalculated analogously with a center of mass calculation) of the openingat the interfacial plane between the hole and the associated volume,from the associated volume to the working electrode, which volume isdelimited on the associated volume side by

-   -   a plane wherein the cross-sectional area of the opening first        time (when moving from the associated volume to the working        electrode) is less than 150% of the smallest cross-sectional        area of the opening,        and delimited on the chamber side by    -   a first plane (when moving from the the associated volume to the        working electrode) comprising a part of the working electrode,        and/or    -   a plane wherein the cross-sectional area of the opening first        time (when moving from the the associated volume to the working        electrode) is more than 150% of the smallest cross-sectional        area of the opening.

The cross-sectional area in this context is understood to be across-sectional area of said opening—without the analyte permeablemembrane—in a plane orthogonal to said path.

The one or more analyte permeable openings have a membrane (where it isunderstood that the membrane may comprise a plurality of separatemembranes, one in each analyte permeable opening in the case of aplurality of analyte permeable openings), but the membrane is analytepermeable. The one or more analyte permeable openings may be at aninterface between the first and second solid element and/or as one ormore through-going holes in the first and/or second solid element.

By ‘membrane’ is understood a membrane material which is placed in theone or more analyte permeable openings and/or in front of the one ormore analyte permeable openings, such as a sheet in front of the one ormore analyte permeable openings, said sheet optionally being a Teflon™sheet. More particularly, the membrane is at least placed in the one ormore analyte permeable opening. The membrane is arranged so as toseparate the associated volume from the volume within the chamber. Morespecifically, the membrane is situated so as to fill or cover the one ormore analyte permeable openings, so as to block passage from theassociated volume to the volume of any substance incapable ofpenetrating through the membrane. It is understood that the membrane mayrefer to a structure, such as a plug in an opening, that separates twofluids, such as a liquid or gas in the associated volume and a liquid inthe chamber. It is understood, that the membrane may refer to a thin,film-like structure that separates two fluids, such as a liquid or gasin the associated volume, and a liquid or gas in the volume. However, itis also understood that the membrane may act as a selective barrier,allowing some species to pass through but not others. It is inparticular understood, that the membrane is permeable to the analyte. Itmay furthermore be understood that the membrane is not permeable toions. The membrane may in particular embodiment comprise, such asconsist of, silicone, such as any one of the silicone sealantsobtainable from Dow Corning with product number 732 or 734. The membranemay be understood to be impenetrable to the liquid electrolyte solutionin the chamber (and the membrane may therefore enable retaining theliquid electrolyte solution in the chamber). It may be understood thatwhile a molecule, e.g., H₂0 in liquid form cannot penetrate themembrane, the same molecule might be able to penetrate the membrane ingaseous form.

‘Working electrode’ is known in the art, and understood to be theelectrode in the electrochemical sensor on which the reaction ofinterest is occurring. It may be understood that the reduction oroxidation of analyte (or a shuttle/mediator molecule) is taking place atthe working electrode.

‘Reference electrode’ is known in the art, and understood to be theelectrode in the electrochemical sensor, which has a stable well-definedelectrochemical potential, and can receive or deliver electrons, from orto working and guard electrode reactions.

In an embodiment there is presented a sensor wherein said chamber (110)is comprising an electrolyte solution. In an embodiment there ispresented a sensor wherein the electrolyte solution is a liquidcomprising ions wherein charge carriers are dissolved ionic compounds.

In an embodiment there is presented a sensor wherein the membrane (124)enables separating liquids on either side of the one or more analytepermeable openings. This may allow for ensuring that the volume in thechamber is not contaminated with liquid from the associated volumeand/or that the liquid in the chamber is not lost into the associatedvolume.

In an embodiment there is presented a sensor wherein the membrane (124)forms a hydrophobic barrier. This may allow for keeping aqueous liquidson either side of the membrane apart. More particularly, for example agas that comes in contact with the sensor first passes through a (smallcapillary-type) opening and then diffuses through a hydrophobic barrier,and eventually reaches the electrode surface. This approach may beadopted to allow the proper amount of analyte to react at the sensingelectrode to produce a sufficient electrical signal while preventing theelectrolyte from leaking out of the sensor.

In an embodiment there is presented a sensor wherein the analytepermeable membrane (124) is a polymer,

-   -   the analyte permeable membrane (124) is passive, and/or    -   the analyte permeable membrane (124) is selective to non-ionic        substances.

In an embodiment there is presented a sensor wherein one or more or allleads are at least partially placed on one or both of the first andsecond solid element at an interface where the first and second solidelement are joined. This may allow for avoiding disadvantageousreactions at the leads.

In an embodiment there is presented a sensor wherein the sensor is aClark-type sensor. By a ‘Clark-type sensor’ may be understood anelectrochemical sensor where a (passive) membrane ensures a separation(such as electrical and ionic) between the associated volume and theworking electrode (and more generally the sensor electrochemistry).Thus, the associated volume does not act as an electrolyte so thereference and the working electrode does not have electrical contactthrough a sample in the associated volume (such as blood). A Clark-typesensor is also known as an amperometric sensor, which is a sensor thatproduces an electrical current as a function of the analyteconcentration (in the associated volume).

In an embodiment there is presented a sensor wherein

-   -   a. the analyte permeable membrane is a polymer, such as an        organic or inorganic polymer, such as silicone, such as        fluorosilicone,    -   b. the analyte permeable membrane is passive, such as not        reacting with the diffusing species (such as the analyte), and    -   c. the analyte permeable membrane is selective to non-ionic        substances.

In another embodiment there is provided an electrochemical sensor,wherein the sensor further comprises a third electrode working as acounter electrode. ‘Counter electrode’ is known in the art andunderstood as an electrode which can deliver or receive electrons (i.e.,current) from the working electrode. The counter electrode may also bereferred to as an auxiliary electrode. An advantage using a counterelectrode may be that less current runs between the working/guardelectrodes and the reference electrode, which enhance the stability ofthe reference electrode.

By ‘guard electrode’ is understood an additional electrode with respectto the working electrode, such as an additional cathode or anode, whichis arranged so as to enable reduction or oxidation of at least somereactants from at least a part of the reservoir region, such asreactants which could otherwise diffuse to the working electrode and bereduced or oxidized at the working electrode. By ‘arranged so as toenable reduction or oxidation of at least some reactants from at least apart of the reservoir region’ may be understood that the guard electrodehas a size large enough and a position sufficiently close to possiblepaths from the reservoir region to the reaction region, so that it mayreduce or oxidize the reactants. An advantage of this may be that saidreactants cannot then cause a false positive signal or noise at theworking electrode. A guard electrode is described in “An oxygenmicrosensor with a guard cathode”, NP Revsbech, Limnol. Oceanogr.,34(2), 1989, 474-478, which is hereby incorporated by reference inentirety.

The guard electrode may be implemented as a thin film at an inner wallof the chamber, such as on the first and/or second solid element. Anadvantage of this may be that the first/and or solid element then doublefunctions as chamber wall and supporting structure for the guard.Another advantage may be, that the guard can be implemented in a waywherein it occupies substantially zero volume in the chamber and causesnot obstruction to diffusion within the chamber. Another possibleadvantage may be that the position of the guard with respect to thechamber is fixed, such as fixed in a robust way.

‘Reactants’ are understood as is common in the art, such as a substancethat is consumed in the course of a chemical reaction. Moreparticularly, as reactants may be understood both (sought-after) analyteand unspecific species. In other words: By reactants from the reservoirregion may be understood any species, which can lead to a reaction atthe working electrode, which could give a signal, which coulderroneously be interpreted as an analyte from the associated volume.

In an embodiment there is presented a sensor wherein said chambercomprising an electrolyte solution. The chamber may comprise (such as befilled with) at least 25 volume %, such as at least 50 volume %, such asat least 75 volume %, such as at least 90 volume %, such as at least 95volume % electrolyte solution, such as 100 volume % electrolytesolution. There may be no gas phase between the working electrode andthe analyte permeable membrane and/or there must be a continuous path ofelectrolyte solution between the working electrode and the analytepermeable membrane. The viscosity of the liquid electrolyte may be lowerthan 10,000 cP (Centipoise) (10 kilo cP).

In a further embodiment there is presented a sensor wherein theelectrolyte solution being a liquid comprising ions wherein chargecarriers are dissolved ionic compounds.

By ‘thin film’ is understood a layer of material having a thicknesswithin a thickness corresponding to an atomic monolayer of the materialto one or more micrometers, such as to 1 micrometer, such as to 2micrometer, such as to 5 micrometer, such as to 10 micrometer, inthickness, where ‘thickness’ may be understood as a length through thematerial along its smallest dimension, which may generally be adimension which is parallel with a surface normal of a surface uponwhich the thin film is placed. It may also be understood that a thinfilm is a structure, where at least a part of it has a primary size(such as width and/or length) in a first and/or second direction, whichfirst and second directions are orthogonal to each other, while asecondary size (such as a length, such as a height or thickness) in athird direction, which is orthogonal to the first direction and thesecond direction, is smaller than the primary size, such as a ratiobetween

-   -   the primary size, and    -   the secondary size,

is at least 10:1, such as at least 100:1, such as at least 1000:1.

The thin film may be placed at the electrochemical sensor by depositionor growth on a solid surface of the sensor, such as the first or secondsolid element. By ‘deposition’ or ‘growth’ is understood any process ofplacing a material on a surface in an additive manner, such as physicaldeposition (e.g., physical vapour deposition (PVD), molecular beamepitaxy (MBE), electron beam evaporation, sputtering, pulsed laserdeposition (PLD), ion beam deposition, cathodic arc deposition(arc-PVD), electro hydrodynamic deposition) or chemical deposition(chemical vapour deposition (CVD), plating, spin coating, atomic layerdeposition (ALD), chemical beam epitaxy). The thin film can also bedeposited on the whole solid surface and thereafter removed, such asetched away, in selected areas.

In an embodiment, there is presented a sensor wherein the membrane (124)is not permeable to ions. By ‘not permeable to ions’ may be understoodsubstantially impermeable to ions, such as impermeable to ions. By‘substantially impermeable to ions’ may be understood impermeable forions in practical circumstances (such as in a context of anelectrochemical sensor, such as a Clark-type sensor). By ‘substantiallyimpermeable to ions’ may in particularly be understood as permeable orless permeable than any silicone or the silicone sealants obtainablefrom Dow Corning with product number 732 or 734. An advantage of havingthe membrane being impermeable to ions may be that it enableselectrically isolating the chamber from the associated volume and/orthat it facilitates maintaining an ionic composition of an electrolyte(such as an electrolyte solution) in the chamber.

In an embodiment, there is presented a sensor wherein a substancemoving, such as diffusing, from

-   -   the most distant part of the reservoir region with respect to        the reaction region,

to

-   -   any point in the reaction region,

would have to pass a point equal to or less than 100 micrometer awayfrom the guard electrode, such 75 micrometer or less, such as 50micrometer or less, such as 25 micrometer or less, such as 10 micrometeror less, such as 5 micrometer or less. By ‘most distant point of thereservoir region with respect to the reaction region’ may be understoodthe point in the chamber, for which the longest path would have to betraversed (such as traversed by substance, such as a reactant, e.g., bydiffusion) in order to get to any point the reaction region. Anadvantage of having the sensor arranged so that the substance getsrelatively close to the guard may be, that it enables the guard to moreefficiently react with the substance before it reaches the reactionregion, where it can react with the working electrode and cause asignal, which could be confused with a signal caused by an analyte fromthe associated volume.

In an embodiment, there is presented a sensor wherein a substancemoving, such as diffusing, from

-   -   the most distant part of the reservoir region with respect to        the reaction region,

to

-   -   any point in the reaction region,

would have to pass a point equal to or less than 10 micrometer, such as5 micrometer or less, away from the guard electrode.

In an embodiment, there is presented a sensor wherein the guardelectrode is arranged so that an electrolyte conductance between workingelectrode and reference electrode is substantially similar, such assimilar, such as substantially identical, such as identical, for thesensor compared to a similar sensor wherein the guard electrode, such asthe electrically conducting part of the guard electrode, has beenremoved. An advantage of this may be that the guard electrode, such asthe electrically conducing part of the guard electrode, does not presentany obstacle to diffusion. Thus, the guard electrode may be close tosubstances passing from the distant part of the reservoir region to thereaction region, yet present no obstacle to diffusion, which in turnenables exchange of electrolyte at the working electrode. By electrolyteconductance being substantially similar, such as similar, may beunderstood that the electrolyte conductance between working electrodeand reference electrode corresponds to 90% or more, such as 95% or more,such as 98% or more, such as 99% or more, compared to an electrolyteconductance between working electrode and reference electrode for asimilar sensor where the guard electrode, such as the conductivematerial of the guard electrode, has been removed.

In an embodiment, there is presented a sensor wherein the workingelectrode is a thin film, and wherein

-   -   the working electrode and    -   the guard electrode

are both placed on the first solid element or are both placed on thesecond solid element. An advantage of this may be that a distancebetween said electrodes is fixed, such as fixed in a very robust way,for example since inertial forces are not likely to be able to moveeither of such thin film electrodes, e.g., if the sensor is accelerated.

In an embodiment, there is presented a sensor wherein the sensorcomprises a plurality of analyte permeable openings. The openings can beplaced in a row and/or in a grid. Alternatively, the openings can beplaced at arbitrary positions. An advantage of having a plurality ofanalyte permeable openings may be, that it increases the area of theopening (which may be beneficial for having a large amount of analytereaching the working electrode, which in turn may yield a larger signaland enhance sensitivity), without increasing the size of the individualholes (where a relatively smaller size each individual opening may bebeneficial for reducing a flow dependence). Thus, having a plurality ofopenings may be beneficial for overcoming the otherwise necessarytradeoff between high sensitivity and stirring sensitivity. Anotherpossible advantage may be that for a given total area the individualopenings may have smaller widths (or diameters in case of circularcross-sections), which may in turn enable that a meniscus of a membranematerial placed in fluid form in the openings has a smaller radius ofcurvature. Another possible advantage may be that for a given total areathe individual openings may have smaller widths (or diameters in case ofcircular cross-sections), which may in turn enable that a meniscus of amembrane material placed in fluid form in the openings has a smallermaximum distance from the point of the liquid surface closest to theassociated volume to the point of the surface furthest away from theassociated volume (as measured in a direction of movement of an analytediffusing from the associated volume to the working electrode along theshortest possible path). This maximum distance may be described as theheight of the meniscus from the bottom in the center to the top pointsat the side.

In an embodiment, there is presented a sensor wherein the first solidelement is joined to the second solid element by bonding, such aspermanent bonding, such as anodic bonding and/or wherein the first solidelement and/or the second solid element comprises at least 20 wt %silicon, such as at least 50 wt % silicon, such as at least 75 wt %silicon, such as at least 99 wt % silicon, such as 100 wt % silicon. By‘bonding’ is understood a method of joining, such as permanently and/orirreversibly joining, two surfaces by chemical and/or physical forces,such as chemical and/or physical bonds. Bonding, such as permanentbonding, can be achieved using any one of anodic bonding, fusionbonding, direct bonding, eutectic bonding and adhesive bonding. Anadvantage of bonding the first and second solid element together may bethat it enables forming in a relatively simple, efficient and compactmanner a fluid tight interface between the first and second solidelement. Another possible advantage of bonding, such as anodic bonding,may be that it enables electrically isolating the electrical connections(leads) to the electrodes by having the leads placed between the firstand second solid element. It may be understood that one or more or allthe leads (which may be thin films) may be placed on one or both of thefirst and second solid element, such as wherein joining (such asbonding) the first and second solid elements may simultaneously embedand encapsulate the leads in the resulting sandwich structure. Anadvantage of anodic bonding may be, that it enables having leads on thefirst and or second solid element, such as on surface of the firstand/or second solid elements which are bonded to the opposite element,where said leads may be of non-zero height above the surface, such asfor example 100 nm.

An advantage of having the first and/or second solid element comprisingsilicon may be that it facilitates that the sensor can be produced viareadily available microfabrication processes, which are applicable forsilicon-based materials.

In an embodiment, there is presented a sensor wherein a plane may bedefined which is parallel with and tangential with a boundary wall ofeach of, such as each and all of:

-   -   The reaction region,    -   the reservoir region, and    -   at least one the one or more analyte permeable openings.

For example, a plane may be parallel and tangential with a surface ofthe first solid element, which is itself planar and serves as a lid ontop of the second solid element wherein a cavity is formed whichcorrespond to at least part of the chamber (with the reaction region andat least part of the reservoir region) and at least one of the one ormore analyte permeable openings. An advantage of this may be that theplanar (first) solid element can be kept very simple and/or that therequirements to alignment of the first solid element can be keptrelatively relaxed. In another example, which may be combined with theprevious example, a cavity is formed in the first and/or second solidelement, which cavity correspond to the chamber (with the reactionregion and at least part of the reservoir region) and at least one ofthe one or more analyte permeable openings, and where a plane may beparallel and tangential with a bottom boundary surface of both thechamber (with the reaction region and at least part of the reservoirregion) and at least one of the one or more analyte permeable openings.An advantage of this may be that it enables forming said cavity in quitesimple way, e.g., by etching to the same depth everywhere, and orplacing, such as depositing, boundary surfaces of the same heighteverywhere. By ‘bottom’ is in this context understood the boundary wallin an element, which is parallel with and opposite a boundary wall onthe opposite solid element.

In an embodiment, there is presented a sensor wherein one or both of:

-   -   The working electrode (104), and    -   The reference electrode (108),

comprise a thin film. An advantage of this may be that all electrodesmay be provided simultaneously, such as in the same process step, e.g.,deposited through a mask. Another possible advantage may be, that itenables providing multiple electrodes of the same kind, such as multipleworking electrodes. It may be understood that in case a plurality ofelectrodes are implemented as thin film electrodes they may all be onthe first solid element or on the second solid element or there may beat least one electrode on the first solid element and at least one otherelectrode on the second solid element. The electrodes may be comprisedof different materials or be comprised of multiple layers, such asincluding one or more layers of material electrodeposited on anothermaterial.

In an embodiment, there is presented a sensor wherein a distance between

-   -   the working electrode

and

-   -   a point in the reaction region which is furthest away with        respect to the working electrode

is 500 micrometer or less, such as 250 micrometer or less, such as 100micrometer or less, such as 50 micrometer or less, such as 25 micrometeror less, such as 10 micrometer or less, such as 5 micrometer or less. Anadvantage of this may be that having a relatively small distance betweenthe working electrode and the point in the reaction region which isfurthest away with respect to the working electrode (where said distanceis understood to be measured as the distance a substance, such as areactant, would have to travel, such as diffuse, from said point to theworking electrode), entails that a period of time from starting thesensor until a steady baseline signal is achieved is relatively small,because said period of time depends on the actual time it takes from asubstance to travel said distance.

In an embodiment, there is presented a sensor wherein a distance between

-   -   the working electrode

and

-   -   a point in the reaction region which is furthest away with        respect to the working electrode

is 50 micrometer or less, such as 25 micrometer or less, such as 10micrometer or less, such as 5 micrometer or less.

In an embodiment, there is presented a sensor wherein an area, such asan area in the chamber or at a wall of the chamber, covered by theworking electrode is equal to or less than 2500 square micrometer, suchas equal to or less than 2000 square micrometer, such as equal to orless than 1500 square micrometer, such as equal to or less than 1000square micrometer, such as equal to or less than 600 square micrometer,such as equal to or less than 250 square micrometer, such as equal to orless than 100 square micrometer, such as equal to or less than 75 squaremicrometer, such as equal to or less than 50 square micrometer, such asequal to or less than 25 square micrometer, such as equal to or lessthan 10 square micrometer. A possible advantage of this may be that thezero current from unspecific reactions on the working electrode isminimized while maintaining the sensitivity. The small size may alsomake it possible to place the guard electrode very close to the analytepermeable opening and thereby minimize the volume of the reactionregion.

In an embodiment, there is presented a sensor wherein a smallest (suchas at the position where the opening is narrowest) total cross-sectionalarea of the one or more analyte permeable openings (122) in across-sectional plane being orthogonal to a direction of movement of ananalyte diffusing from the associated volume to the working electrodealong the shortest possible path is equal to or less than 0.25 squaremillimeter, such as equal to or less than 0.10 square millimeter, suchas equal to or less than 0.05 square millimeter, such as equal to orless than 0.01 square millimeter, such as equal to or less than 0.005square millimeter, such as equal to or less than 0.0025 squaremillimeter, such as equal to or less than 2500 square micrometer, suchas equal to or less than 1000 square micrometer. A possible advantage ofhaving a relatively small smallest total cross-sectional area of the oneor more analyte permeable openings (122) may be that the smallness ofthis area facilitates little evaporation from the chamber and littleinflux of, e.g., contaminants or water vapour into the chamber. Anotherpossible advantage may be that it facilitates drawing only a lowcurrent, e.g., less than 1 nA at the working electrode, which may inturn facilitate extended lifetime and/or low stirring sensitivity.

In an embodiment, there is presented a sensor wherein a ratio(A_(min,opening)/A_(min, WE-Ref)) between

-   -   A first smallest total cross-sectional area (A_(min,opening)) of        the one or more analyte permeable openings (122) in a        cross-sectional plane being orthogonal to a direction of        movement of an analyte diffusing from the associated volume to        the working electrode along the shortest possible path, and    -   A second smallest total cross-sectional area (A_(min, WE-Ref))        of the chamber along a shortest possible path of a species        diffusing from the working electrode (WE) to the reference        electrode (Ref), said second smallest cross-sectional area        (A_(min, WE-Ref)) being in a cross-sectional plane being        orthogonal to a direction of movement of a species diffusing        from the working electrode (WE) to the reference electrode (Ref)        along the shortest possible path,

is equal to or less than 1, such as equal to or less, such as equal toor less than 0.5, such as equal to or less than 0.1, such equal to orless than 0.05, such as equal to or less than 0.01, such as equal to orless than 0.001. An advantage of this embodiment may be that it ensuresthat for an opening area (A_(min,opening)) which allows an amount ofanalyte to enter the chamber an be reduced or oxidated at the workingelectrode and thereby generate reaction products—at least the same areais available for species diffusing to and from the reference (from tothe working electrode). An possible advantage of this may be, that itmay ensure or facilitate that there will be little or no buildup ofreaction products at the working electrode.

In an embodiment, there is presented a sensor wherein the first solidelement comprises, such as consists of, silicon and/or wherein thesecond solid element comprises, such as consists of, borosilicate.

In an embodiment, there is presented a sensor wherein the first solidelement and the second solid element are bonded together optionally withanodic bonding.

In an embodiment, there is presented a sensor wherein the analytepermeable membrane comprises, such as consists of, a polymer, such as aninorganic polymer, such as silicone, such as fluorosilicone.

In an embodiment, there is presented a sensor wherein the analytepermeable membrane enables separating liquids, such as aqueoussolutions, on either side of the one or more analyte permeable openings.

In an embodiment, there is presented a sensor wherein the shortestdistance from

-   -   any point on the working electrode,

through the one or more analyte permeable openings and the analytepermeable membrane to

-   -   a point on an opposite side of the analyte permeable membrane        with respect to the working electrode,

is equal to or less than 300 micrometer, such as equal to or less than275 micrometer, such as equal to or less than 250 micrometer, such asequal to or less than 225 micrometer, such as equal to or less than 200micrometer, such as equal to or less than 100 micrometer, such as equalto or less than 50 micrometer. A possible advantage of having thisdistance being relatively small may be that it enables reducing aresponse time of the sensor.

In an embodiment, there is presented a sensor wherein the shortestdistance (239) from

-   -   any point on the working electrode (104),    -   through the one or more analyte permeable openings (122) and the        analyte permeable membrane (124) to    -   a point on an opposite side of the analyte permeable membrane        with respect to the working electrode (104),

is equal to or less than 100 micrometer. A possible advantage of havingthis distance being relatively small may be that it enables reducing aresponse time of the sensor.

In an embodiment, there is presented a sensor wherein the one or moreanalyte permeable openings are placed at least partially, such as fully,between the first solid element and the second solid element. A possibleadvantage of this may be that it allows freedom in design of the one ormore analyte permeable openings in a relatively simple manner, becausethey can for formed as a cavity in the first and or second solid elementbefore joining of the first and second solid elements. This may enabledesign and manufacture, such as relatively simple manufacture, ofopenings with relatively complicated geometries (sizes and or shapes),such as elongated (high aspect ratio), curved, zig-zag shaped walls.

By ‘the one or more analyte permeable openings are placed at leastpartially between the first solid element and the second solid element’may be understood that an opening borders on both the first solidelement and the second solid element (such as is abutting, such astouching, both the first solid element and the second solid element),such as being enclosed only partially by each of the first solid elementand the second solid element. An interface between the first solidelement and the second solid element, such as a bonding interface, maydefine an interface plane, and it may be understood that an analyte maytravel from the associated volume to the chamber, such as to the workingelectrode, via a path confined in a plane being parallel with, such asidentical to, said interface plane. A distance from the first solidelement to the second solid element in a direction orthogonal to theinterface plane and in the chamber between the first solid element andthe second solid element (such as a maximum distance at the position ofthe working electrode) is 50 micrometers or less, such as 25 micrometersor less, such as 10 micrometers or less, such as 5 micrometers or less.A possible advantage of this may be that it facilitates a small chambervolume and ensures that any analyte comes close to the workingelectrode.

In an embodiment, there is presented a sensor wherein the one or moreanalyte permeable openings, such as the one or more analyte permeableopenings without the analyte permeable membrane, are arranged so that adistance from any point in at least one cross-sectional plane to thenearest point of a wall of said opening is 25 micrometer or less, wheresaid cross-sectional plane is orthogonal to a direction of movement ofan analyte diffusing from the associated volume to the working electrodealong the shortest possible path.

A possible advantage of this may be that the sensor may furthermore beadvantageous for having a relatively low stirring sensitivity, becausethe relatively small distance from the any point in the opening to thewall of said opening may facilitate that depletion of analyte in frontof the opening is minimized. The stirring sensitivity S_(sen) is definedas:

S _(sen)=(c _(INF) /c ₀)−1,

where c_(INF) is the concentration at infinite flow velocity in theassociated volume and c₀ is the concentration measured without flow.

A possible advantage of a small analyte permeable opening may be that itenables that a meniscus of a membrane material placed in fluid form inthe opening has a smaller maximum distance from the point of the liquidsurface closest to the associated volume to the point of the surfacefurthest away from the associated volume (as measured in a direction ofmovement of an analyte diffusing from the associated volume to theworking electrode along the shortest possible path). This maximumdistance may be described as the height of the meniscus from the bottomin the center to the top points at the side. This smaller maximumdistance in turn renders the position of the end and/or beginning of themembrane material better defined, which in turn improves manufacturingtolerances, such as reducing inter-sensor variations.

In an embodiment, there is presented a sensor which endures adifferential pressure of 4 bar or more, such as 5 bar or more, such as10 bar or more, such as bar or more, such as 50 bar or more, such as 100bar or more. In general, an advantage of the present sensor may be, thatit may be constructed so as to endure exposure to high pressure(relative to atmospheric pressure). For example, the sensor may endure adifferential pressure (on either side of the analyte permeable membrane,such as the difference in pressure one one side of the membrane withrespect to the pressure on the other side, such as the difference inpressure in the associated volume with respect to the chamber) of atleast 4 bar. The present inventors have made the insight that theability of the sensor to endure high pressure may be achieved in aplurality of different ways, including having areas of the one or moreanalyte permeable openings being small and/or by having the membranebeing relatively long (such as relative to said areas) and/or whereinone or more or all boundary walls of the one or more analyte permeableopenings have a non-rectilinear shape.

In an embodiment there is presented a sensor wherein an angle between aboundary wall of the one or more analyte permeable openings at the endof the one or more analyte permeable openings which faces the chamberand an abutting wall of the chamber is more than 270 degrees, such asmore than 271 degrees, such as more than 275 degrees, such as more than280 degrees, such as more than 285 degrees, such as more than 290degrees, such as more than 300 degrees, such as more than 315 degrees,such as more than 330 degrees, such as 345 degrees or more. A possibleadvantage of this may be that it enables increasing the capillary forcesat the end of the one or more analyte permeable openings.

In an embodiment, there is presented a sensor wherein the sensorcomprises one or more additional electrodes, such as:

-   -   A scavenger electrode, such as a scavenger electrode placed        between the one or more analyte permeable openings and the        working electrode,    -   An additional working electrode in the reaction region, such as        wherein the working electrode and the additional working        electrode are placed between the one or more analyte permeable        openings and the guard electrode, such as placed in parallel or        in series with respect to a direction of movement of an analyte        diffusing along the shortest possible path from the gas        permeable opening to the working electrode.

A possible advantage of having a scavenger electrode (which may functionin a similar manner to the guard electrode) placed between the one ormore analyte permeable openings and the working electrode may facilitatethat interfering substances from the associated volume may be renderedharmless in terms of the measurements at the working electrode by thescavenger electrode. A possible advantage of having an additionalworking electrode may be that one working electrode may be placed infront of the other (with respect to the one or more analyte permeableopenings) and operated in a time-varying manner so that the signal onthe other working electrode depends on the time-varying operation, sothat the signal on the working electrode in combination with knowledgeof the time-variation may be used to realize very low detection limits.Another possible advantage of having an additional working electrode maybe that one working electrode may be placed in front of the other (withrespect to the one or more analyte permeable openings), so that anysignal on the other working electrode may be interpreted as anindication that a range linear detection of the first working electrodeis exceeded. Another possible advantage of having multiple workingelectrodes may be that it enables measuring simultaneously differentanalytes (e.g., one type of analyte for each working electrode). Incertain sensor embodiments it can be advantageous to place a scavengerchemical or electrode in a separate chamber in front of the analytepermeable membrane in order to remove an interfering species.

According to a second aspect of the invention, there is presented use ofa sensor according to the first aspect for sensing an analyte in anassociated volume.

According to an embodiment, there is presented use of the sensor whereinthe analyte is sulfide. According to an embodiment, there is presenteduse of the sensor for measuring sulfide in a sewer and/or in wastewater. According to an embodiment, there is presented use of the sensorfor measuring sulfide in natural gas and/or biogas. According to anembodiment, there is presented use of the sensor for measuring sulfidein natural gas and/or biogas during a desulfurization process.

The first and second aspect of the present invention may each becombined with any of the other aspects. These and other aspects of theinvention will be apparent from and elucidated with reference to theembodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The electrochemical sensor according to the invention will now bedescribed in more detail with regard to the accompanying figures. Thefigures show one way of implementing the present invention and is not tobe construed as being limiting to other possible embodiments fallingwithin the scope of the attached claim set.

FIG. 1A depicts an electrochemical sensor for sensing an analyte.

FIG. 1B shows the sensor with a third solid element.

FIG. 2 shows a sensor with a plurality of analyte permeable openings.

FIG. 3 shows a top-view of a sensor corresponding to the side view inFIG. 1A.

FIG. 4 shows a top-view with a plurality of analyte permeable openings.

FIG. 5 shows a perspective view of a sensor.

FIG. 6 illustrates movement of a substance.

FIG. 7 depicts a sensor comprising an additional electrode.

FIG. 8 depicts an additional working electrode.

FIGS. 9-10 illustrate a detailed process of manufacturing a sensor

FIG. 11-12 shows signals as obtained with a sensor.

FIGS. 13-14 shows (light) microscope images of a sensor.

FIGS. 15-16 shows scanning electron microscope (SEM) images.

FIG. 17 shows the first and second solid with a third solid element.

FIG. 18 shows schematic drawing of embodiment with multiple workingelectrodes.

FIG. 19 shows image of embodiment with multiple working electrodes.

DETAILED DESCRIPTION OF AN EMBODIMENT

FIG. 1A depicts an electrochemical sensor 100 for sensing an analyte inan associated volume 106, the sensor comprising

-   -   a first solid element (126),    -   a second solid element (128) being joined to the first solid        element,    -   a chamber (110) being placed at least partially between the        first solid element and the second solid element, said chamber        comprising        -   a reaction region (130), and        -   a reservoir region (132) being connected with the reaction            region,    -   wherein an analyte permeable opening (122) connects the reaction        region (130) with the associated volume (106), such as said        analyte permeable opening forming a diffusion barrier between        the associated volume and the chamber, and wherein the        electrochemical sensor (100) further comprises    -   an analyte permeable membrane (124) in said analyte permeable        opening, such as a silicone membrane, such as a membrane which        enables separating liquids (such as aqueous solutions) on either        side of the analyte permeable opening,    -   a working electrode (104) in the reaction region,    -   a reference electrode (108), and    -   a guard electrode (109) arranged so as to enable reduction or        oxidation of at least some reactants from at least a part of the        reservoir region, such as reactants which could otherwise        diffuse to the working electrode (104) and be reduced or        oxidized at the working electrode, wherein the guard electrode        comprises a thin film, such as a thin film placed on an inner        wall of the chamber    -   wherein the one or more analyte permeable openings are placed at        least partially, such as fully, between the first solid element        and the second solid element,    -   and wherein the one or more analyte permeable openings are        arranged so that a distance from any point in at least one        cross-sectional plane to the nearest point of a wall of said        opening is 25 micrometer or less, where said cross-sectional        plane is orthogonal to a direction of movement of an analyte        diffusing from the associated volume to the working electrode        along the shortest possible path.

FIG. 1A furthermore shows an electrical connection pad 140 forelectrical connection to the working electrode 104 and guard electrode109, and an electrolyte opening 136 for filling of electrolyte into thecavity between the first and second solid elements. The dotted line 134indicates the interface between reaction region 130 and reservoir region132. The working electrode (104) is a thin film, and the workingelectrode and the guard electrode (109) are both placed on the secondsolid element (128). The first solid element (126) is joined to thesecond solid element (128) by anodic bonding and the first solid elementis made from a silicon wafer. The second solid element comprises glass,such as Pyrex glass.

FIG. 1B shows the sensor of FIG. 1A wherein an third solid element 129,such as part of a housing, has been placed adjacent to the first solidelement, and forming fluid-tight interface between these elements bymeans of an O-ring. An advantage of this may be that the chamber maythen be enlarged by being also partially confined by the third solidelement, thus more electrolyte can be kept in the chamber, which in turnincreases lifetime of the sensor. The figure furthermore showselectrical wires 141 connecting the electrical connection pad toperipheral electronics and a wire reference electrode 108 inserted as awire in the third solid element 129. In another embodiment one or bothof the working electrode 104 and the reference electrode comprise a thinfilm. For example, the reference electrode may instead of the wirereference electrode 108 be added as thin film electrode on the firstand/or second solid element, which may be advantageous for simplifyingproduction.

FIG. 2 shows a sensor which comprises a plurality 222 of analytepermeable openings. The figure furthermore shows the analyte permeableopenings being formed in one of the solid elements, which differs withrespect to the embodiment depicted in FIGS. 1A-1B where the analytepermeable opening is formed at the interface between the first andsecond solid element.

FIG. 2 furthermore indicates a distance 237 between

-   -   the working electrode 104

and

-   -   a point 238 (such as encircled by the dashed circle 238) in the        reaction region which is furthest away with respect to the        working electrode

is 500 micrometer or less, such as 250 micrometer or less, such as 100micrometer or less, such as 50 micrometer or less, such as 25 micrometeror less, such as 10 micrometer or less, such as 5 micrometer or less.

FIG. 2 furthermore indicates that the shortest distance 239 from

-   -   any point on the working electrode 104, through the analyte        permeable opening and the membrane to    -   a point on an opposite side of the membrane with respect to the        working electrode,

is equal to or less than 300 micrometer, such as equal to or less than200 micrometer, such as equal to or less than 100 micrometer, such asequal to or less than 50 micrometer.

FIG. 3 shows a top-view of a sensor which corresponds to the side viewdepicted in FIG. 1A. In FIG. 3 the layout of the working electrode 104and guard electrode 109 can be seen, and it can furthermore be seen thatelectrical connections may be integrated in the structure of the sensoroutside of the chamber 110. Note that the dotted line denotes theposition of the chamber (which may correspond to a cavity in the firstand or second solid element), and the first and second solid element areforming a joining, such as bonding, interface outside the dotted line,such as encapsulating the thin film leads of the electrodes. In thepresent embodiments, the thin film of the electrodes is similar to thethin film of the leads, but the electrodes are exposed to the chamberwhereas the leads are encapsulated in the sandwich structure of thefirst and second solid element. The length and width of the workingelectrode 104 in the chamber 110 in the present embodiment is 25micrometer×100 micrometer, corresponding to an area covered of 2500square micrometer. FIG. 3 thus depicts a sensor wherein an area covered,such as an area projected onto the wall of the chamber upon which it isplaced, by the working electrode is equal to or less than 2500 squaremicrometers.

FIG. 4 shows a top-view of a sensor similar to the top-view depicted inFIG. 3, except that the single analyte permeable opening 122 in FIG. 3is replaced with a plurality 422 of analyte permeable openings.

FIG. 5 shows a perspective view of a sensor corresponding to thetop-view depicted in FIG. 4, wherein cavities have been formed in firstsolid element 526 which cavities may the correspond to inter alia theplurality 422 of analyte permeable openings at the interface between thefirst and second solid elements when the first solid interface is joinedto planar solid element 528. The figure also shows electrolyte opening536, and a corner of an electrical connection pad is just visible on thesecond solid element behind the first solid element.

FIG. 6 illustrates that a substance moving (as indicated by arrow 642),such as diffusing, from

-   -   the most distant part 648 (such as encircled by dashed circle        648) of the reservoir region with respect to the reaction        region,

to

-   -   any point in the reaction region,

would have to pass a point equal to or less than 100 micrometer awayfrom the guard electrode (said distance indicated by double-headed arrow644), such 75 micrometer or less, such as 50 micrometer or less, such as25 micrometer or less, such as 10 micrometer or less, such as 5micrometer or less.

FIG. 6 furthermore indicates that the guard electrode (609) is arrangedso that an electrolyte conductance is substantially similar, such assimilar, for the sensor compared to a similar sensor wherein the guardelectrode has been removed. Since the guard electrode is implemented asa thin film electrode, which occupies substantially no volume, itenables that the cross-sectional area—which is related to and increaseswith the electrolyte conductance between working electrode and referenceelectrode—can be kept relatively high, which in turn enablescontinuously having a sufficient electrolyte supply, such as ions in theelectrolyte, to the working electrode from the reservoir region.

FIG. 6 furthermore indicates that a plane 646 may be defined which isparallel with and tangential with a boundary wall of each of:

-   -   The reaction region,    -   The reservoir region,    -   The analyte permeable opening.

FIG. 7 depicts a sensor comprising an additional electrode 704 a placedbetween the analyte permeable opening and the working electrode 704 b. Aguard electrode 709 is also depicted in FIG. 7.

FIG. 8 depicts working electrode 804 a and an additional workingelectrode 804 b in the reaction region, wherein the working electrodeand the additional working electrode are placed between the analytepermeable opening and the guard electrode. In FIG. 8 they are placed inparallel, but they could also be placed in series (similar to electrodes704 a-b in FIG. 7) with respect to a direction of movement of an analytediffusing along the shortest possible path from the analyte permeableopening to the working electrode. A guard electrode 809 is also depictedin FIG. 8.

FIGS. 9-10 illustrate a detailed process of manufacturing a sensoraccording to an embodiment of the invention.

FIG. 9 comprises side views.

FIG. 10 comprises top-views.

In step 1a the cavities are etched in a silicon wafer (<100>, 4-inch,350 micrometer, double side polished). First, the Si wafer is treated inbuffered hydrogen fluoride (BHF) for 30 seconds. In step 1b a 1.5 umAZ5214e Novolac resist is spun on the wafer and a part of the chamber isetched anistropically 5 micrometer into the Si by deep reactive-ionetching (DRIE). In step 1c thereafter, through-holes are etched usingthe same method, but using 10 micrometer resist. The wafer was attachedto a carrier wafer using Krystal Bond™ before performing the deep etch.In step 1d an insulating layer of 100 nm SiO₂ is formed by thermaloxidation. In step step 2a-2c 100 nm Pt thin-film electrodes aredeposited on a Pyrex wafer using 2.2 micrometer AZ5214e Novolac asimage-reversal as lift-off resist. Before physical vapor deposition ofPt, the same areas may optionally (step 2b) be etched by 50 micrometerin BHF to recess the electrodes. 2 nm Ti is deposited before Pt toincrease the adhesion. In step 3 the Si wafer and the Pyrex wafer arejoined by anodic bonding at 350° C. using 600 volts. In step 4 thesilicone membrane material is filled into the channels and cured. Instep 5 the wafers as diced with blue foil covering the openings in theSi wafer. After dicing the chip is attached to an external electrolytechamber in which the reference electrode wire is placed. The device isfilled with electrolyte. Remaining air bubbles are removed byincubation/boiling in vacuum at room temperature.

FIG. 11 shows in the left figure a signal as obtained with a sensoraccording to an embodiment of the invention as a function of time, wherea concentration of H₂S in the associated volume has been increased insteps (the steps corresponds to additions of 20 micromolar andsubsequently 3 additions of 100 micromolar). In the right figure, thesignal is plotted as a function of the H₂S concentration in theassociated volume. It may be derived from the signal that a responsetime of the sensor is approximately 3 seconds (corresponding to the timeit takes from the change in analyte concentration in the associatedvolume is increased in a step function and until the signal reaches 90%of its final settled value). In other embodiments the response time maybe lower, such as 0.3 seconds, such as 0.1 second.

FIG. 12 shows start-up signals as obtained with a sensor according to anembodiment of the invention as a function of time, respectively, withand without the guard electrode being employed for electrochemicalreactions. It may be seen that the presence of the guard electrodeenables rapidly achieving a low-base line signal.

FIG. 13 shows a (light) microscope image of a sensor corresponding inthe left figure (which is magnification of the right figure) to the topview seen, e.g., in schematic FIG. 3, where the working electrode 1304and guard electrode 1309 can be seen. In the right figure, alsoelectrolyte opening 1336 and electrical connection pads 1340 areindicated. In the right side figure, an on-chip reference electrode 1308is depicted. A trough-going hole is placed in the silicon above thecircular part of the electrode.

FIG. 14 shows a (light) microscope image of an end of the sensorcorresponding to the end with the plurality of analyte permeableopenings 422 in FIG. 5. The image furthermore shows the first solidelement 526 and the second solid element 528.

FIG. 15 shows a scanning electron microscope (SEM) image similar to the(light) microscope image in FIG. 14.

FIG. 16 is another SEM image similar to the image in FIG. 15, but withhigher magnification. The analyte permeable openings and the analytepermeable opening imaged comprises a silicone membrane.

FIG. 17 shows an embodiment wherein the first and second solid elementsare integrated in a third solid element with a larger volume forenlarging the chamber (with respect to a volume of the chamber betweenthe first and second solid elements). The first and second solid elementjoined together is indicated by the arrow, and inserted in a housingwith a printed circuit board and connections for power and datatransmission. The printed circuit board contains a voltage source and acurrent meter sensitive to currents in the range of picoamperes.

FIG. 18 shows a schematic drawing of embodiment with multiple workingelectrodes. The left hand side shows inlets with multiple workingelectrodes (4 pairs of two sequentially arranged working electrodes) anda guard electrode. The right hand side shows working electrodes incompartments with no inlets, which working electrodes can be used totrack non-analyte related effects, such as noise, temperature andstability. The left hand side inlets (4 inlets) may have similarmembrane lengths to ensure easy comparison (if everything is in order,the signals should then be similar for sensors at different openings) ordifferent lengths, for example the upper compartment may have relativelyshort membranes in the openings and the the lower compartment may haverelatively long membranes in the openings (where ‘relatively’ refers tothe other compartment of upper and lower), which may ensure longerlifetime.

FIG. 19 shows an image of embodiment with multiple working electrodes(such as corresponding to the schematic in FIG. 18).

Although the present invention has been described in connection with thespecified embodiments, it should not be construed as being in any waylimited to the presented examples. The scope of the present invention isset out by the accompanying claim set. In the context of the claims, theterms “comprising” or “comprises” do not exclude other possible elementsor steps. Also, the mentioning of references such as “a” or “an” etc.should not be construed as excluding a plurality. The use of referencesigns in the claims with respect to elements indicated in the figuresshall also not be construed as limiting the scope of the invention.Furthermore, individual features mentioned in different claims, maypossibly be advantageously combined, and the mentioning of thesefeatures in different claims does not exclude that a combination offeatures is not possible and advantageous.

1. An electrochemical sensor for sensing an analyte in an associatedvolume, the sensor comprising: a first solid element, a second solidelement being joined to the first solid element, a chamber being placedat least partially between the first solid element and the second solidelement, said chamber comprising: a reaction region, and a reservoirregion being connected with the reaction region, wherein one or moreanalyte permeable openings connect the reaction region with theassociated volume, and wherein the electrochemical sensor furthercomprises: an analyte permeable membrane in said one or more analytepermeable openings, a working electrode in the reaction region, areference electrode, and a guard electrode configured to enablereduction or oxidation of at least some reactants from at least a partof the reservoir region, wherein the guard electrode comprises a thinfilm. 2-39. (canceled)
 40. The sensor according to claim 1, wherein themembrane is not permeable to ions.
 41. The sensor according to claim 1,wherein the shortest distance from: any point on the working electrode,through the one or more analyte permeable openings and the analytepermeable membrane to a point on an opposite side of the analytepermeable membrane with respect to the working electrode, is equal to orless than 100 micrometer.
 42. The sensor according to claim 1, wherein asubstance moving from: the most distant part of the reservoir regionwith respect to the reaction region, to any point in the reactionregion, passes a point equal to or less than 100 micrometer away fromthe guard electrode.
 43. The sensor according to claim 1, wherein saidchamber comprises an electrolyte solution.
 44. The sensor according toclaim 43, wherein the electrolyte solution is a liquid comprising ions,wherein charge carriers are dissolved ionic compounds.
 45. The sensoraccording to claim 1, wherein the membrane enables separating liquids oneither side of the one or more analyte permeable openings.
 46. Thesensor according to claim 1, wherein the membrane forms a hydrophobicbarrier.
 47. The sensor according to claim 1, wherein: the analytepermeable membrane is a polymer, the analyte permeable membrane ispassive, or the analyte permeable membrane is selective to non-ionicsubstances.
 48. The sensor according to claim 1, wherein one or more orall leads are at least partially placed on one or both of the first andsecond solid elements at an interface where the first and second solidelement are joined.
 49. The sensor according to claim 1, wherein thesensor is a Clark-type sensor.
 50. The sensor according to claim 1,wherein the sensor is a microsensor.
 51. The sensor according to claim1, wherein a substance moving from: the most distant part of thereservoir region with respect to the reaction region, to any point inthe reaction region, passes a point equal to or less than 10 micrometeraway from the guard electrode.
 52. The sensor according to claim 1,wherein the guard electrode is configured such that an electrolyteconductance between working electrode and reference electrode issubstantially similar for the sensor compared to a similar sensor,wherein the guard electrode has been removed.
 53. The sensor accordingto claim 1, wherein the working electrode is a thin film, and wherein:the working electrode and the guard electrode are both placed on thefirst solid element or are both placed on the second solid element. 54.The sensor according to claim 1, wherein the sensor comprises aplurality of analyte permeable openings.
 55. The sensor according toclaim 1, wherein the first solid element is joined to the second solidelement by bonding or wherein the first solid element and/or the secondsolid element comprises at least 20 wt % silicon.
 56. The sensoraccording to claim 1, wherein a plane may be defined, which is parallelwith and tangential with a boundary wall of each of: The reactionregion, The reservoir region, and At least one of the one or moreanalyte permeable openings.
 57. The sensor according to claim 1, whereinone or both of: The working electrode, and The reference electrode,comprise a thin film.
 58. The sensor according to claim 1, wherein adistance between: the working electrode and a point in the reactionregion, which is furthest away with respect to the working electrode is500 micrometer or less.
 59. The sensor according to claim 1, wherein adistance between: the working electrode and a point in the reactionregion, which is furthest away with respect to the working electrode is50 micrometer or less.
 60. The sensor according to claim 1, wherein anarea covered by the working electrode is equal to or less than 2500square micrometer.
 61. The sensor according to claim 1, wherein asmallest total cross-sectional area of the one or more analyte permeableopenings in a cross-sectional plane is orthogonal to a direction ofmovement of an analyte diffusing from the associated volume to theworking electrode along the shortest possible path is equal to or lessthan 0.25 square millimeter.
 62. The sensor according to claim 1,wherein a ratio (A_(min,opening)/A_(min, WE-Ref)) between: A firstsmallest total cross-sectional area (A_(min,opening)) of the one or moreanalyte permeable openings in a cross-sectional plane is orthogonal to adirection of movement of an analyte diffusing from the associated volumeto the working electrode along the shortest possible path, and A secondsmallest total cross-sectional area (A_(min, WE-Ref)) of the chamberalong a shortest possible path of a species diffusing from the workingelectrode (WE) to the reference electrode (Ref), said second smallestcross-sectional area (A_(min, WE-Ref)) being in a cross-sectional planebeing orthogonal to a direction of movement of a species diffusing fromthe working electrode (WE) to the reference electrode (Ref) along theshortest possible path, is equal to or less than
 1. 63. The sensoraccording to claim 1, wherein the first solid element comprises siliconor wherein the second solid element comprises borosilicate.
 64. Thesensor according to claim 1, wherein the first solid element and thesecond solid element are bonded together.
 65. The sensor according toclaim 1, wherein the analyte permeable membrane comprises a polymer. 66.The sensor according to claim 1, wherein the analyte permeable membraneenables separating liquids on either side of the one or more analytepermeable openings.
 67. The sensor according to claim 1, wherein anangle between a boundary wall of the one or more analyte permeableopenings at the end of the one or more analyte permeable openings, whichfaces the chamber and an abutting wall of the chamber is more than 285degrees.
 68. The sensor according to claim 1, wherein the shortestdistance from any point on the working electrode, through the one ormore analyte permeable openings and the analyte permeable membrane to apoint on an opposite side of the analyte permeable membrane with respectto the working electrode, is equal to or less than 300 micrometer. 69.The sensor according to claim 1, wherein the one or more analytepermeable openings are placed at least partially between the first solidelement and the second solid element.
 70. The sensor according to claim1, wherein the one or more analyte permeable openings are configuredsuch that a distance from any point in at least one cross-sectionalplane to the nearest point of a wall of said opening is 25 micrometer orless and, wherein said cross-sectional plane is orthogonal to adirection of movement of an analyte diffusing from the associated volumeto the working electrode along the shortest possible path.
 71. Thesensor according to claim 1, wherein the sensor endures a differentialpressure of 4 bar or more.
 72. The sensor according to claim 1, whereinthe sensor comprises one or more additional electrodes.
 73. The sensoraccording to claim 72, wherein the sensor comprises one or moreadditional scavenger electrodes.
 74. The sensor according to claim 72,wherein the sensor comprises one or more additional working electrodesin the reaction region.
 75. A method for sensing an analyte in anassociated volume comprising contacting the sensor of claim 1 with ananalyte provided in a volume of liquid or gas and sensing said analyte.76. The method according to claim 75, wherein the analyte is sulfide.77. The method of claim 75, wherein the analyte is sulfide provided in avolume of sewer or waste water.
 78. The method of claim 75, wherein theanalyte is sulfide provided in a volume of natural gas or biogas. 79.The method of claim 78, wherein the volume of natural gas or biogas isproduced in a desulfurization process.